#main {
    width: 600px;
    height: 200px;
    margin: 0 auto;
    border: solid crimson 10px;
}
.sub {
    width: 600px;
    height: 200px;
    margin: 0 auto;
    border: solid chartreuse 10px;
    box-sizing: border-box;
}
#relative {
    position: absolute;
    height: 100%;
    border: solid chartreuse 1px;
    top: 10px;
    bottom: 50px;
}